The present invention relates to a magnetic recording system, and more particularly a magnetic recording system having a recording density of 2 gigabits or higher and a thin film magnetic recording medium with low noises for realizing such system.
Demands for large capacity of a magnetic recording system are increasing more and more nowadays. Conventionally, an inductive head has been used which utilizes a voltage change caused by a change of a magnetic flux with time. This inductive head performs both read and write. Recently, use of a composite head has expanded rapidly which uses different heads for read and write by introducing a magnetoresistive head having a higher sensitivity as a read head. The magnetoresistive head utilizes the phenomenon that the electrical resistance of the head element changes with a leakage flux change of a medium. A head having a much higher sensitivity constituted of a plurality of magnetic layers laminated between a plurality of non-magnetic layers laminated between those magnetic layers is now under development which utilizes a very large magenetoresistance change (giant magnetoresistive effect or spin valve effect). This giant magnetoresistive effect is an electrical resistance change to be caused by a change in relative directions of magnetization of a plurality of magnetic layers interposed between non-magnetic layers. And the relative direction change is caused by leakage fluxes of a recording medium.
Magnetic layers of magnetic recording media presently used in practice are made of alloy whose main components are Co such as CoCrPt, CoCrTa and CoNiCr. Such Co alloy has a hexagonal closed packed structure (hcp structure) having a c-axis as a magnetic easy axis. It is therefore preferable that an in-plane magnetic recording medium has the crystallographic orientation having the c-axis oriented along the in-plane direction. However, such an orientation is unstable so that it cannot be formed if Co alloy is directly deposited on a substrate. The (100) plane of Cr having a body centered cubic structure (bcc structure) has a good lattice matching with the (11.0) plane of Co alloy. By using this good lattice matching, first an underlayer of Cr having the (100) plane is fabricated on a substrate, and a Co alloy layer is epitaxially grown on the Cr underlayer to thereby form the (11.0) plane having the c-axis oriented in the in-plane direction. In order to further improve the crystal lattice matching at the interface between the Co alloy magnetic layer and the Cr underlayer, a second element is added to Cr to increase an interstitial distance. The (11.0) crystallographic orientation of Co alloy therefore increases further and coercivity can be increased. Examples of these techniques are to add V, Ti, or the like as disclosed in JP-A-62-257618 and JP-A-63-197018.
Factors necessary for high density magnetic recording include low noises as well as high coercivity of recording media. Media noises are mainly caused by an irregular zig-zag pattern formed in magnetization transition regions between recording bits. It is necessary to smooth these transition regions in order to reduce media noises. It is known that fine magnetic crystal grains and uniform crystal grain sizes are effective for reducing media noises. To this end, it is effective to make fine and uniform crystal grains of the underlayer. The above-described known techniques increase the lattice constant of the underlayer by adding a second element to the Cr underlayer, but do not make fine and uniform crystal grains of the underlayer. Therefore, although the above techniques are effective for increasing coercivity, they are not effective for reducing media noises.
Significant requisites for magnetic disk media are improvement of shock resistance. This shock resistance improvement is a very significant issue from the viewpoint of reliability of magnetic disk media, particularly for magnetic disk drives mounted on recent portable information apparatuses such as note type personal computers. Instead of using an Al alloy substrate with an NiP plated surface (hereinafter called an Al alloy substrate), using a glass substrate with a reinforced surface or a crystallized glass substrate can improve the shock resistance of magnetic disk media. As compared to conventional Al alloy substrates, the glass substrate has a smoother surface so that it is suitable for high density recording because it is effective for reducing a flying space between the magnetic heads and the medium. The glass substrate is, however, associated with some problems such as insufficient adhesion relative to the substrate and permeation of impurity ions on the substrate or adsorbed gas on the substrate surface into the Cr alloy underlayer. Of these problems, the film adhesion property in particular is degraded if the glass substrate is heated as reported in J. Vac. Sci. Technol. A4(3), 1986, at pp. 532 to 535.
Countermeasures for these problems include fabricating a film such as a metal film, an alloy film and an oxide film between the glass substrate and Cr alloy underlayer (JP-A-62-293512, JP-A-2-29923, JP-5-135343).
As compared to an Al alloy substrate, a glass substrate of an in-plane magnetic recording medium has worse electromagnetic conversion characteristics at high linear recording density regions. The reason for this is as in the following. A Cr alloy underlayer fabricated on a glass substrate directly or via a film made of one of metals or its alloy described in the above conventional techniques, does not display strong (100) orientation compared to it is fabricated on an Al alloy substrate. Therefore, the crystal plane other than the (11.0) plane of the Co alloy magnetic layer grows parallel to the substrate surface and the in-plane orientation of the c-axis as, which is magnetic easy axis becomes small. From this reason, coercivity lowers and a read output at high linear recording density lowers. Furthermore, if the glass substrate is used, crystal grains in the magnetic layer become bulky than using an Al alloy substrate, and the crystal grain size dispersion becomes larger by about 20 to 30%. These are main reasons to increased media noises and degraded electromagnetic conversion characteristics of media using the glass substrate. JP-A-4-153910 discloses that the size of crystal grains of a magnetic layer can be suppressed from becoming bulky and the magnetic characteristics can be improved if an amorphous film being made of Y and one of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and the like is inserted between the glass substrate and underlayer.
With this method, however, although the size of crystal grains of the magnetic layer may be reduced to some degree, the in-plane components of the magnetic easy axis reduce and this is not sufficient for a magnetoresistive head to realize a high recording density of 2 gigabits or more per square inches. Furthermore, the effects of reducing grain size distribution are rare and good electromagnetic conversion characteristics cannot be obtained.
Although a magnetoresistive head is suitable for high density recording because it has a very high read sensitivity, the sensitivity relative to noises also becomes high. Therefore, in-plane magnetic recording media with low noises are required more than ever.
In order to reduce media noise and obtain good electromagnetic conversion characteristics even at high recording density, it is necessary to make crystal grain size fine and reduce grain size distribution, without degrading the hcp (11.0) orientation of the Co alloy magnetic layer.
Furthermore, even if samples of a magnetic disk drive is manufactured for use with a combination of a low noise in-plane magnetic medium and a high sensitivity magnetoresistive head, sufficient electromagnetic conversion characteristics cannot be always obtained. This may be ascribed to independent developments of magnetic heads and in-plane magnetic recording media, and to insufficient consideration about the way of head-disk combination for high recording density of the magnetic disk drive.
It is an object of the invention to solve the above problems and provide a magnetic recording system having high reliability and a recording density of 2 gigabits per square inches or higher and an in-plane magnetic recording medium of low noises suitable for high density recording.
According to one aspect of the invention, a magnetic recording system is provided which comprises: an in-plane magnetic recording medium having a magnetic layer fabricated on a single underlayer or on a plurality of underlayers fabricated on a substrate; a driver unit for driving the in-plane magnetic recording medium in a write direction; a magnetic head having a read unit and a write unit; means for moving the magnetic head relatively to the in-plane magnetic recording medium; and read/write signal processing means for reading an output signal from the magnetic head and writing an input signal to the magnetic head, wherein the read unit of the magnetic head is a magnetoresistive head and the single underlayer or at least one of the plurality of underlayers of the medium is made of Co-containing amorphous material or fine crystal material, or is made of alloy material having at least one element selected from a group consisting of Cr, Mo, V and Ta as the main components and containing at least one element selected from a group consisting of B, C, P and Bi.
It is another object of the present invention to eliminate adhesion defects which are likely to occur when a film is fabricated after a glass substrate is heated. If the adhesion defects can be eliminated, the conditions of fabricating a film can be broadened to reduce noises of a recording medium, and the substrate can be heated immediately before a film is fabricated. It is therefore possible to remove impurity gas adsorbed on the substrate surface and to improve reproductivity of the magnetic film characteristics.
As described earlier, as compared to a conventional NiP plated Al alloy substrate, a glass substrate degrades the electrical characteristics because the in-plane component of the c-axis, which is the magnetic easy axis of a Co alloy magnetic layer is small and the crystal grain size is large. Crystal grains of the magnetic layer are epitaxially grown on crystal grains of the underlayer. Therefore, the orientation and size of crystal grains of the magnetic layer are generally affected by the crystal grain size and surface morphology of the underlayer. Basing upon these knowledge, the present inventors manufactured various in-plane magnetic recording media by changing materials of underlayers, layer structures, film thicknesses, film fabricating conditions, and the like, and evaluated the read/write characteristics in combination with composite magnetic heads having an inductive head as a write unit and a magnetoresistive head as a read unit. It has been found that the electrical characteristics can be improved by using a multi-layer structure of underlayers, and by inserting a new underlayer (hereinafter called a first underlayer) made of Co-containing amorphous material or fine crystal material, between the substrate and the bcc structure underlayer made of Cr alloy or the like which improves the crystal lattice matching with the magnetic layer. The details of this will be given in the following.
Amorphous means that a clear peak cannot be observed by X-ray diffraction measurements or that a clear refraction spot or ring cannot be observed by electron beam refraction measurements but a halo refraction ring can be observed. Fine crystal means that the crystal grain size is smaller than that of the magnetic layer, and preferably has an average grain size of 8 nm or smaller.
The components of the Co-containing amorphous or fine crystal material of the multi-layer underlayer are not limited specifically if the co-containing alloy contain any elements which form Co-containing amorphous or fine crystal material with Co. If the first underlayer made of Co-containing amorphous or fine crystal material is fabricated on a glass substrate, crystal grains of the underlayer constituted of Cr alloy or the like having the bcc structure and formed on the first underlayer (hereinafter called a second underlayer) are made fine and at the same time the (100) plane of the bcc structure is likely to grow parallel to the film surface. Therefore, crystal grains of the Co alloy magnetic film grow with its magnetic easy axis being parallel to layer the film plane direction and the grain size becomes small. So the coercivity can be increased and noises can be reduced. If amorphous or fine crystal material not containing Co is used, the crystal grain size of the magnetic layer becomes somewhat small in some cases. However, as shown in Embodiment 7, if the Co-containing amorphous or fine crystal material is used, the crystal grains are made finer and the crystal grain size dispersion becomes smaller. This may be ascribed to that the Co-containing amorphous or fine crystal material forms uniform and fine projections on the film surface and crystal grains of the second underlayer grow by using this uniform and fine projections as crystal seeds.
It is preferable that the structure of the first underlayer is an amorphous structure. The amorphous structure can realize media with lower noises than the fine crystal structure, because crystal grains of the second underlayer and magnetic layer are finer. However, even the structure of the first underlayer is the fine crystal structure which has an average crystal grain size of 8 nm or smaller, good electrical characteristics can be obtained. Although the fine crystal structure generates slightly higher noises, a read output at a high recording density becomes large because the magnetic layer has a strong (11.0) orientation. Therefore, this structure is suitable for use with a head having relatively high noises.
Specific material of the first underlayer is preferably alloy of Co and at least one additive element selected from a group consisting of Ti, Y, Zr, Nb, Mo, Hf, Ta, W, Si and B (hereinafter called the first group), or compound of Co and oxide of the additive element selected from the first group. The content of the additive element is preferably in the range from 5 at % or higher and 70 at % or lower. If the concentration of the additive element is smaller than 5 at %, crystal grains of the magnetic layer become larger than when the second underlayer is fabricated on a glass substrate directly, whereas if it is larger than 70 at %, components whose c-axis rises from the surface of the magnetic film become large and the vertical magnetic anisotropy becomes strong. Therefore, this outer range is not preferable. It is particularly preferable to use Zr, Ta or W in the group as the first additive element, because the in-plane orientation components of the magnetic easy axis become strong.
Magnetization of the first underlayer may affect the read/write characteristics. The first underlayer is therefore preferably made of non-magnetic material. However, it has been confirmed from our studies that if a product of a residual magnetic flux density and a film thickness of the first underlayer is 20% or smaller of a product of a residual magnetic flux density and a film thickness of the magnetic layer, there is no practical problem. If the product of the residual magnetic flux and the film thickness of the first underlayer exceeds 20% of the product of the residual magnetic flux and the film thickness of the magnetic layer, the base line of an output signal obtained by using an magnetoresistive head fluctuates and low frequency noises increase. This outer range is therefore not preferable. In order to avoid this, it is effective to make the first underlayer thin, to increase the concentration of the additive element, or to add a second additive element. Cr, V, Mn (hereinafter called the second group) or the like as the second additive element is effective because it lowers magnetization greatly.
The second underlayer is preferably Cr or alloy of Cr and at least one element selected from Ti, Mo and V. The second underlayer may be constituted of two layers having the bcc structure. If the second underlayer is made of alloy having as the main components at least one element selected from a group consisting of Cr, Mo, V and Ta (hereinafter called the third group) and containing at least one element selected from a group consisting of B, C, P and Bi (hereinafter called the fourth group), crystal grains of the underlayer become fine and the grain size becomes uniform. Therefore, crystal grains of the magnetic layer fabricated on the underlayer also become fine and uniform so that media noise can further be reduced.
FIG. 1 shows the dependence of normalized media noise and S/N on Brxc3x97t of media using a Crxe2x80x9415 at % Ti underlayer or a underlayer added with 5 at % B, i.e., Crxe2x80x9414.3 at % Tixe2x80x945 at % B. These media were manufactured by changing film structures and process conditions so that almost the same coercivities were obtained. A numeral and symbols affixed to the top of each element indicate the concentration of each element represented by an atomic percentage (at %). The normalized media noise is defined as media noise normalized by an output of an isolated read signal and a track width. In the following description, the media noise is evaluated by using this normalized media noise. The normalized noise is reduced by about 15% and the S/N is enhanced for the media using the CrTiB underlayer compared to the media using the CrTi underlayer at any values of Brxc3x97t is used. Similar to the CrTi underlayer, the CrTiB underlayer has the bcc structure and the (100) orientation, and the hcp (11.0) orientation of the Co alloy magnetic layer is not degraded.
FIG. 2 shows the relationship between the concentration of B added to the Crxe2x80x9415 at % Tixe2x80x94B underlayer and the normalized media noise. The media noise is reduced by the addition of B. However, if the B concentration exceeds 20 at %, the noise reduction effect disappears. This may be ascribed to degradation of crystal structure of the underlayer and hence degraded that of the magnetic layer. If the B concentration is smaller than 1 at %, finesse and uniformity of crystal grains are insufficient and the noise reduction effect is poor.
The noise reduction effect has been also confirmed when an element selected from the fourth group excepting B is added. Similar to the addition of B, the addition of P reduced noises considerably. On the other hand, the addition of C considerably enhanced the coercivity and coercivity squareness S*, and the addition of Bi allowed to fabricate media excellent in corrosion resistance. The concentration of these added elements is preferably 1 at % or higher and 20 at % or lower, and in the range from 2 to 8 at % in particular, media with low noises could be obtained.
In order to improve lattice matching at the interface between the underlayer and Co alloy magnetic layer and to improve the magnetic characteristics, an element such as V, Mo and the like as well as Ti may be added. It has been confirmed that if an element selected from the fourth group is added to the CrV alloy or CrMo alloy underlayer, crystal grains is made fine and uniform like it is added to CiTi alloy and the noise reduction effect occurs. As compared to the media having the CiTi or CrMo alloy underlayer containing an added element selected from the fourth group, the media having the CrV underlayer containing added element selected from the fourth group element, in particular, has good overwrite characteristics. If the fourth group element is added to the CrMo underlayer, a strong bcc (100) orientation and good crystal sturcutre are realized even at a relatively low temperature. Therefore, the carbon protective film can be fabricated at a low temperature with an improved film quality so that media having better CSS performance can be realized. From the synthetic comparison, it can be said as follows. Particularly, media having an underlayer of B-added CrTi alloy added with Ti for improving lattice matching, have the magnetic easy axis of Co alloy, strongly oriented in the in-plane direction and crystal grains are made fine and uniform to a large extent. Therefore, media can be realized having excellent read and write characteristics and satisfying both the high resolution and the low noise.
The magnetic layer may use alloy having Co as its main components, such as CoCrPt, CoCrPtTa, CoCrPtTi, CoCrTa and CoNiCr. In order to obtain a high coercivity, it is particularly preferable to use Co alloy which contains Pt. Magnetic alloy containing rare earth elements, such as SmCo and FeSmN, may also be used. It is known that an SmCo alloy film is made of very fine crystal grains. However, since the magnetic interaction between these crystal grains is strong, each crystal grain is not independent and discrete magnetically. If this film is fabricated on the underlayer having the bcc structure, an aggregation of SmCo alloy crystal grains fabricated on each underlayer crystal grain is considered to function as one magnetic unit. According to the invention, the first underlayer made of Co-containing amorphous or fine crystal material makes fine crystal grains of the second underlayer. Therefore, the magnetic unit of SmCo alloy is also made fine so that media noises can be reduced. The magnetic layer may be structured as a single layer or a plurality of layers interposed with intermediate layers. In this case, the thickness t of the magnetic layer as recited in claims means a total thickness of magnetic layers.
As to the magnetic characteristics of the magnetic layer, it is preferable to set the coercivity to 1.8 kilo oersteds or higher measured by applying a magnetic field in the write direction and the product of a residual magnetic flux density Br and a film thickness t to 20 gaussxc2x7micron or larger and 140 gaussxc2x7micron or smaller, because good read and write characteristics are obtained in the recording density range of 2 gigabits per square inches. It is not preferable to make the coercivity smaller than 1.8 kilo oersteds, because an output at a high recording density (200 kFCI or higher) lowers. If the product Brxc3x97t becomes larger than 140 gaussxc2x7micron, the resolution lowers, whereas if the product Brxc3x97t becomes smaller than 20 gaussxc2x7micron, a read output becomes small. This outer range is therefore not preferable.
A protective layer for the magnetic layer is formed by depositing carbon to a thickness of 5 to 30 nm and a lubricating layer made of, for example, adsorptive perfluoroalkyl-polyether is formed to a thickness of 1 to 20 nm. In this manner, a magnetic recording medium capable of high density recording with high reliability can be obtained. It is preferable to use as the protective layer a carbon film added with hydrogen, a film made of compound such as silicon carbide, tungsten carbide, (Wxe2x80x94Mo)xe2x80x94C and (Zrxe2x80x94Nb)xe2x80x94N, or a film made of a mixture of such compound and carbon, because the slide resistance and corrosion resistance can be improved. It is also preferable to form fine projections on the surface of the deposited protective layer through plasma etching using a fine mask or the like, to form protrusions of different phases on the protective layer surface by using targets of compounds or mixed materials, or to form fine projections on the protective layer surface by heat treatment, because the contact area between the head and medium can be reduced so that adhesion of the head to the medium surface during CSS operations can be avoided.
It has been found that if the first underlayer made of Co-containing amorphous or fine crystal material of this invention is used, the adhesion property is good even if the first underlayer is fabricated after the glass substrate is heated, as in the case where the substrate is not heated. This good adhesion property can be supposed as resulting from a strong bond between cobalt which is the main components of the first underlayer and silicon or oxygen of the glass substrate. If compound of Co and the oxide of additive element selected from the first group is used, the adhesion property relative to the glass substrate can be improved further, and this compound is particularly suitable for the case where the flying amount (spacing between the magnetic head and medium) of the magnetic head slider is small and contact therebetween is likely to occur. As described above, according to the present invention, a specific layer for improving the adhesion property is not required. However, an underlayer may made of low melting point metal such as Al and Ag, alloy of such metal, or metal compound be fabricated between the substrate and the first underlayer in order to form projections on the medium surface and improve CSS characteristics.
If the second underlayer is made of alloy having as the main components at least one element selected from the third group consisting of Cr, Mo, V and Ta and containing at least one element selected from the fourth group consisting of B, C, P and Bi, the first underlayer may be a layer made of alloy having the Co-containing amorphous or fine crystal material or an orientation control layer of Ta or the like for making the second underlayer have the (100) orientation.
If metal such as Ti, Zr and Cr or its oxide is fabricated between the orientation control layer and the glass substrate, the adhesion property relative to the glass can be improved and diffusion of adsorbed gas on the substrate, impurity ions in the glass, or the like can be suppressed so that good magnetic characteristics are obtained.
It has been also confirmed that similar to the glass substrate, use of the underlayer stated above can make crystal grains of the magnetic layer fine and uniform even if an Nixe2x80x94P plated Al alloy substrate is used.
It is preferable to set the space (shield space) to 0.35 xcexcm or smaller between two shield layers sandwiching the magnetoresistive sensor unit of the magnetoresistive head of the in-plane magnetic recording system of this invention. The reason of setting such a space is that if the shield space is 0.35 xcexcm or larger, the resolution lowers and a signal jitter becomes large.
The magnetoresistive head is constituted of a magnetoresistive sensor having a plurality of conductive magnetic layers and conductive non-magnetic layers disposed between these magnetic layers. This sensor generates a large resistance change when magnetization directions of each magnetic layers are relatively changed by an external magnetic field, which is called the giant magnetoresistive effect or spin value effect. In this case, the read output signal can be enhanced further and a magnetic recording system can be realized which has a high recording density of 3 gigabits per square inches and a high reliability.